Bubbles and bed expansion in low pressure fluidization

Abstract

Fluidization at low pressure has significant benefits over traditional fluidized beds for the fine chemical industry. Yet there is a lack of knowledge of bubbling and bed behaviour under the slip flow regime (low pressure), and this presents a hindrance to the optimization of this process. In this work, the fundamental aspects of gas bubbles and bed expansion are investigated for a wider range of operating pressure, flow rates and bed aspect ratio. Typically, rising individual bubbles undergo a series of breakups and coalesce as they rise in the bed. The lowering of pressure below 14,884 Pa was found to inhibit this behaviour indicating a qualitative indication of the impact of pressure on the bubble dynamics. The bubbles formed at low pressure were larger in average size, greater in number at the inlet and have a higher aspect ratio. All these factors were directly related to the expansion of gas due to the lowering of operating pressure. Under a controlled environment with single rising bubbles, the gas expansion was predictable using the ideal gas equation. However, where the pressure is low enough (≤ 24,231 Pa) the inhomogeneous fluidization impacted the predictability and trend of the bubbling zone and bed expansion. Despite this, the overall consumption of fluidizing mass was magnitudes lower to generate similar sizes of bubbles at atmospheric pressure. This has a direct impact on the usage of fluidizing gas while maintaining similar gas-solid mixing behaviour. Overall, the benefits of low pressure can be obtained at moderate pressure between 49,130 pa and 24,231 Pa at AR 3 with homogeneous fluidization.